This invention pertains to heat pipes, and more particularly to heat pipes employing thin film boiling.
A great deal of research has been conducted in recent years on heat pipes, sparked by extra-terrestrial applications which offer a very cold environment for the condenser and the heat pipe does not have to pump against the gravitational field of the earth. An exemplary of this research is exhibited in A Collection of Technical Papers delivered, and later published, at the third international heat pipe conference at Palo Alto, Calif. on May 22-24, 1978 by the American Institute of Aeronautics and Astronautics, Inc. Most of this research was directed at certain phenomena which effect heat pipe operation.
Axial groove heat pipes have become the industry standard dispite the fact that the conductance of this type heat pipe is far from optimum. The popularity of this type pipe is probably due to the ease of fabrication. Because of the pool boiling characteristic of this type pipe other parameters which influence the functioning of the pipe need not be controlled as precisely as in the case of thin film boiling. Very high heat transfer coefficients are associated with very thin liquid films. One method of achieving thin film boiling in a heat pipe is to present the meniscus of the fluid to the heated surface of the evaporator. In other words, the meniscus would be inverted from that typically found in the axial groove heat pipe. However, it is readily acknowledged that the influence parameters, indicated above, need be more precisely controlled when the meniscus is presented to the heated surface to avoid a burn out. A burn out may be defined as the transition from a completely wet evaporator to a partially dry evaporator. Avoiding a burnout is critical as it represents the practical heat transfer limit of the pipe beyond which it ceases to function. Some of these influence parameters seriously affect the shape of the meniscus. As a practical matter, the pumping power, which is determined by the capillary structure, must be as great as the pressure drop to the capillary structure and the wick plus any head that needs to be overcome e.g. the difference in elevation between the ends of the heat pipe. Capillary pressure is determined by the surface tension of the working fluid and the maximum possible curvature of the menisci in the capillary structure according to the classical Young-Laplace relation. Mechanical equilibrium at the liquid-vapor interface is critical. Excesses of pressure on either the vapor side of the interface or the liquid column hydrostatic pressure cause the menisci to either recede in or to extend out though the capillary screen. Elements must be provided within the heat pipe to control critical influence parameters to control the shape of the meniscus between these two extremes for efficient heat transfer.